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United States Patent |
5,739,883
|
Chen
,   et al.
|
April 14, 1998
|
Manufacturing method and structure for aligner of liquid crystal display
Abstract
This invention is directed to a manufacturing method and a new structure of
a LCD aligner. The aligner is obtained by using transparency gradually
changed mask for lithographic exposure. The LCD aligner made by this
method has the function of both multidomain division and that of the
retardation film. The advantages of LCD aligners made this way includes:
simpler fabrication process, less cost, improved contrast ratio, reduced
interference color, etc. Besides enhancing the color resolution of the
screen, it also extends the viewing angle of the screen.
Inventors:
|
Chen; Kun-Ti (Tainan, TW);
Ho; Bing-Ming (Chiayi, TW)
|
Assignee:
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Nan Ya Technology Corporation (Taipei, TW)
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Appl. No.:
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705731 |
Filed:
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August 30, 1996 |
Current U.S. Class: |
349/124; 349/128; 349/129 |
Intern'l Class: |
G02F 001/133.7 |
Field of Search: |
349/123,124,126,128,129,132,136
428/1
430/5,20
|
References Cited
U.S. Patent Documents
4725517 | Feb., 1988 | Nakanawatari et al. | 349/124.
|
5438421 | Aug., 1995 | Sugawara et al. | 349/124.
|
5446569 | Aug., 1995 | Iwai et al. | 349/124.
|
5473455 | Dec., 1995 | Koike et al. | 349/124.
|
5486403 | Jan., 1996 | Ishitaka et al. | 428/1.
|
5504604 | Apr., 1996 | Takatori et al. | 349/123.
|
Foreign Patent Documents |
4-037719 | Feb., 1992 | JP | 349/129.
|
Primary Examiner: Sikes; William L.
Assistant Examiner: Duong; Tai V.
Attorney, Agent or Firm: Bacon & Thomas
Claims
What is claimed is:
1. A method for making a LCD aligner which comprises the following steps:
(1) forming a pre-etched aligner layer on an electrode substrate;
(2) coating a photoresist layer on the surface of the pre-etched aligner
layer;
(3) employing a photomask which has a gradually changing transparency line
pattern to form a photoresist pattern with different vertical depths;
(4) using vertical non-selective etching to remove part of said photoresist
layer and pre-etched aligner layer;
(5) removing all the remaining photoresist.
2. The method according to claim 1, wherein said pre-etched aligner layer
is selected from the group consisting of polyimide and silicon nitride
(SiN)x .
3. The method according to claim 1, wherein said photomask has at least two
domains, each domain is mapping to a color pixel and the transparency of
line patterns on said photomask of each domain is changing gradually along
the longitude direction at the same rate, but at a different rate for each
different domain.
4. A method for making a LCD aligner which comprising the following steps:
(1) coating a photo-resist layer onto an electrode substrate;
(2) employing a photomask to expose said photoresist layer into at least
two domains at once;
(3) developing the exposed photoresist to form pretilt angle lines of said
LCD aligner wherein said photomask has at least two domains, each domain
is mapping to a color pixel, and the transparency of line patterns on said
photomask of each domain is changing gradually along the longitude
direction at the same rate, but at a different rate for each different
domain.
Description
BACKGROUND OF THE INVENTION
1.Field of the Invention
This present invention relates generally to a method for forming a
liquid-crystal-display (LCD) aligner, and in particular, to the formation
of a LCD aligner with pretilt angles which are patterned by employing a
mask with gradually changing transparency for lithographic exposure and
etching processing. This method can dramatically reduce the number of
process steps in making the LCD aligner and hence reduce the overall
manufacturing cost of producing LCDs.
2.Descriptions of the Prior Art
Flat display technology is taking various forms today, including the Field
Emission Display (FED), Plasma Display (PD) and Liquid Crystal Display
(LCD). The LCD technology is the best developed among these. Its
importance can be inferred from the geometrical growth of yearly sales
revenue of products containing LCDs. The so called Thin Film Transistor
LCD (TFT LCD) have good qualities which allow it to compete with Cathode
Ray Tube (CRT) containing devices. However, one problem associated with
LCD's is their limited viewing angle, while CRTs are notoriously bulky and
heavy.
A conventional TFT LCD has a large number of liquid crystal molecules piled
up between top and bottom aligners, the arrangement of the liquid crystal
molecules is subject to change when an electric field is applied to the
liquid crystal molecules which force the molecules to change their
orientation which controls the amount of light passing through the
molecules. The operation principle of an LCD is described in the following
paragraph.
As shown in FIG. 1 (A), incident light A is polarized after passing the
polarizer B when no voltage is applied across the two electrodes. The
liquid crystal molecules C rotate this polarized light 90 degrees to pass
through a second polarizer D, which is 90 degrees out of phase with the
polarizer B. This successful passage of light results in a bright spot
represented by the arrowhead in FIG. 1(A).
However, as shown in FIG. 1(B), the liquid crystal molecules are forced to
pile up by the electric field generated when a voltage is applied across
the two electrodes. Therefore, the liquid crystal molecules change their
orientation and will not rotate the light and allow it to pass through the
second polarizer D and this phenomenon results in a black spot, that is,
the absence of transmitted light.
The main axes of the liquid crystal molecules align with a 90 degree
rotation under the effect of aligners. Because the ditch (alignment)
direction of the top and bottom aligners are orthogonal to each other, the
confined liquid crystal molecules will stack up rotationally to fit the
different ditch or aligner direction of the upper and lower alingners.
Consequently, aligners play a key role in the operation of an LCD.
Aligners are conventionally formed by vacuum sputtering or a rubbing
method. The rubbing method is currently the more popular method used in
the mass production of aligners. As shown in FIG. 2, the rubbing method
produces grooves or ditches (G) in one direction only, which is the
rubbing direction. This is shown in FIG. 2(B). However, if the ditches for
each pixel are formed in one rubbing direction only, there will be
vertical viewing angle dependence problem when viewing the LCD. That is,
the light transmission ratios are different with applied voltage higher
than 2 V at different viewing points (or angles) A1, A2 and A3, as shown
in FIGS. 3(A) and 3(B). Thereafter, the viewer will experience
nonequivalent contrast as well as a decrease in sharpness at different
viewing angles, as shown in FIG. 3(C). Moreover, this figure also shows
the lack for symmetry and wide viewing angle.
The manufacture of two or multi-domain aligners was developed to overcome
this problem. As shown in FIG. 4(A), each pixel has two domains, H and I,
with different or opposite rubbing directions of the aligner ditches. Due
to the mutual compensation for viewing characteristics provided by the two
domains, in the vertical direction, the LCD will give distinct images in a
wide viewing range. The LCD using two or multi-domain aligners showed
tremendous improvement in terms of contrast, as shown in FIG. 4(B). But
the method requires multiple rubbing steps, as shown in FIG. 4(C), which
is both time consuming and costly.
Another problem in LCD manufacture is the compensation for multi-frequency
light. This problem arises from the use of multi-frequency light such as
white light as the light source. When the different frequency light passes
through the first polarizer, it incidents into the liquid crystal
molecules in the same polarizing direction. After passing through the
liquid crystal molecules, the polarizing direction of different frequency
light will be rotated to different direction. After the different
frequency light with different polarizing frequency passes the second
polarizer, the different frequency light will have different intensity. As
a solution to this problem, a method which employs a retardation film
which compensates the difference of rotating angles and improves color
contrast, as shown in FIG. 5(A), is used. This also helps widen the
viewing angle, as shown in FIG. 5(B).
Related technology is described in Okabe, "Wide Viewing Angle TFT-LCDs,"
Fujitsu Limited, page 105, ASID 93 and Kaneko et al., "Invited Address:
Wide-Viewing-Angle Improvements for AMLCDs,"NEC Corp., published on page
265, SID 93 Digest.
SUMMARY OF THE INVENTION
In view of foregoing, and in order to reduce the added expense of LCD
production, it is a primary object of the present invention to provide a
reliable and easy to use process for forming the LCD aligner. The process
steps are described as following:
(1) depositing a layer of polyimide or SiNx onto the LCD electrodes as a
pre-etched aligner layer;
(2) coating a layer of photoresist onto the pre-etched aligner layer;
(3) making a photo-mask that has a gradually changing transparency line
pattern;
(4) exposing the photoresist with mentioned mask, therefore this
photoresist was exposed with lights of varied intensity;
(5) developing the photoresist, therewith using non-selective
Reactive-Ion-Etching (RIE) etchback to make an equal depth etching in the
vertical direction, then removing part of the photoresist and aligner
material;
(6) removing all the remaining photoresist to obtain the aligner.
Another object of this invention is to provide another method to improve
the making of the LCD aligner. The process is described as follows:
(1) coating the LCD electrode with a layer of transparent photoresist as
the pre-etched layer for the aligner;
(2) making a photo-mask that has a gradually changing transparency line
pattern;
(3) exposing the photoresist with the aligner line mask, therefore this
photoresist was exposed with lights of varied intensity;
(4) removing part of the resist by developing to obtain the aligner.
It is another object of this invention to find a new structure of the LCD
aligner to compensate rotation angle difference for multi-frequency light
without employing a retardation film.
BRIEF DESCRIPTION OF THE DRAWINGS
In the accompanying drawings forming a material part of this description,
there is shown: FIGS. 1(A)-(B) show the LCD operation mechanism diagrams;
1(A) shows that bright spot is obtained without applied voltage; 1(B)
shows that black spot is obtained with applied voltage.
FIGS. 2(A)-(B) are the illustration diagram of the LCD rubbing method, the
arrow shows the direction of the rubbing.
FIGS. 3(A)-(C) shows the drawback of single domain LCD aligner. 3(A) shows
the transparency of light measured from different angle A1, A2 and A3;
3(B) shows the transparency ratio unevenness at different angles; 3(C)
shows asymmetry of color contrast and viewing angles.
FIG. 4(A) is the cross-sectional representation of the two domain or
multi-domain LCD aligner; FIG. 4(B) shows the color contrast as a function
of viewing angle; FIG. 4(C) shows the conventional rubbing method for
making two-domain or multi-domain LCD aligner.
FIG. 5(A) is the illustration diagram of the LCD with one or two layers of
retardation film; FIG. 5(B) is the plot of the color contrast as a
function of viewing angle for LCD aligners with one, two and no
retardation film(s);
FIGS. 6(A)-(V) are the illustration diagrams to disclose the LCD process
proposed in the present invention.
FIG. 7 is the top view of the LCD aligner structure made by the method of
this invention.
FIG. 8 is the cross-sectional representation of the LCD aligner structure
made by the method of this invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The invention disclosed herein is directed to a method for making LCD
aligner, as shown in FIG. 6. The process steps are described as follows:
(1) As shown in FIG. 6(A), a conventional electrode 1 is provided, and a
polyimide or SiNx layer 2 having a thickness of 500 to 2000 angstroms is
formed on the substrate of the electrode. The exact thickness of this
layer depends on the specific use in subsequent processing and
application. This layer acts as the aligner layer after the etching
process, therefore it is referred as pre-etched aligner layer.
(2) As shown in FIG. 6(B), a layer of photoresist 3 was coated onto the
mentioned pre-etched aligner layer and has a thickness between about
5000.ANG. to 12000.ANG..
(3) FIG. 6(C) shows the forming process of the photomask used to etch the
bottom aligner. First, the area on the mask mapping to R.G. B (red, green
or blue) color pixel is divided into two domains, labeled domain 4 and
domain 5, and second, each domain is disposed with parallel lines 6 to
define the aligner ditches. The dimensions of each line pattern is 50
um.times.5 um.
(4) Two embodiments are disclosed as follows:
Embodiment 1
As shown in FIG. 6(D), the variation rates of light transparency for each
lines on the mask at domain 4 and domain 5 are the same, and both can be
illustrated by FIG. 6(D). In domain 4, the light transparency of starting
point 7 with transparency of 90% is decreased 0.6% for each 0.5 micron of
movement to the left. The light transparency at point 8 is 30%. In domain
5, the light transparency of starting point 9 with transparency of 30%
increases 0.6% for each 0.5 micron movement to the left. The light
transparency at point 10 is 90%. The cross-sectional view of light
transparency variation for each lines on the mask is shown in FIG. 6(E).
Embodiment 2
As shown in FIG. 6(F), the variation rates of light transparency for each
line on the mask at domain 4 and domain 5 are the same, and both can be
illustrated by FIG. 6(F). In domain 4, the light transparency of starting
point 11 with transparency of 90% is decreased 0.6% for each micron of
movement to the left. The light transparency at point 12 is 30%. In domain
5, the light transparency of starting point 13 with transparency of 78%
decreases 0.6% for each 0.5 micron movement to the left. The light
transparency at point 14 is 48%. The cross-sectional view of light
transparency for each line on the mask is shown in FIG. 6(G).
(5) Using the above mentioned photomask to expose and develop the
photoresist. Since the lines in each pixel have varying transparency,
exposed lines receive varying light intensity which varies photoresist
thickness after the developing. Then the aligner layer area for each pixel
will have two domains with different directions after the remaining
photo-resist is removed. FIG. 6(H) is the cross-sectional view of the LCD
electrode after the exposure and develop using the mask from the first
embodiment of this invention. FIG. 6(I) is the cross-sectional view of the
LCD electrode after the exposure and development by using the mask from
the second embodiment of this invention.
(6) Thereafter applying the non-selective Reactive-Ion-Etching (RIE) to
make an equal depth etching in the vertical direction of the LCD aligner.
Finally remove the remaining photoresist afterwards to achieve the line
pattern desired. FIG. 6(J) is the cross-sectional view of the resultant
LCD aligner structure made by using the mask from the first embodiment of
this invention. The pixels have symmetric pretilt angle 12; FIG. 6(K) is
the cross-sectional view of the resultant LCD aligner structure made by
using the mask from the second embodiment of this invention. The pixels
have two different pretilt angles 12.
(7) Two different ways for the top aligner photomask are provided as
followed:
To operate in embodiment 1, as shown in FIG. 6(L), in domain 5, the light
transparency of starting point 17 with transparency of 90% is decreased by
0.6% for each 0.5 micron movement to the left. The light transparency at
point 18 is 30%; in domain 4, the transparency of starting point 15 with
transparency of 30% is increased by 0.6% for each 0.5 micron movement to
the left. The light transparency at point 16 is 90%. The cross-sectional
view of light transparency variation for each line on the mask is shown in
FIG. 6(M);
To operate in embodiment 2, the variation rates of light transparency for
each line on the mask at domain 4 and domain 5 are the same, and both can
be illustrated by FIG. 6(N). In domain 4, the light transparency of
starting point 19 with transparency of 48% is increased by 0.6% for each
0.5 micron movement to the left. The light transparency at point 20 is
78%; in domain 5, the light transparency of starting point 21 with
transparency of 30% is increased by 0.6% for each 0.5 micron movement to
the left. The light transparency at point 22 is 90%. The cross-sectional
view of light transparency variation for each lines on the mask is shown
in FIG. 6(O);
(8) Using the photomask for top aligner of this invention to expose the top
LCD electrode 1 coated with aligner material and photoresist. As shown in
FIGS. 6(P) and 6(R). Then using non-selective Reactive-Ion-Etching (RIE)
etchback to make an equal height etching in the vertical direction of the
LCD aligner, and remove the photoresist afterwards to achieve the line
pattern desired, as shown in FIGS. 6(Q) and 6(S);
(9) FIG. 6(T) shows the mask for the top aligner. The lines have different
rotation angle for light of different color, so the direction of the lines
on the mask should be made for pixels of different color. The rotation
angle of light of certain frequency can be calculated using the following
formula:
T=(1+.mu..sup.2).sup.-1 Sin.sup.2 ›.theta.(1+.mu..sup.2).sup. 1/2!
T: transparency % for LCD
.mu.:.pi..delta..DELTA.n/.theta..lambda.
.theta.: rotation angle of Liquid Crystal molecules
.lambda.: wave length of light
.DELTA.n=Ne -No, Ne: extraordinary index; No: Ordinary index
D: distance between aligners
If T=0 then sin.sup.2 ›.theta.(1+.mu..sup.2).sup.1/2 !=0
.theta.(1+.mu..sup.2).sup.1/2 =.pi.
.theta..sup.2 (1+.mu..sup.2)=.pi..sup.2
.theta..sup.2 +.theta..sup.2 (.pi..delta..DELTA.n/.theta..lambda.).sup.2
=.pi..sup.2
.theta..sup.2 =.pi..sup.2 ›1+(d.DELTA.n/.lambda.).sup.2 !
.theta..sub.red =144.degree. .theta..sub.green =127.degree.
.theta..sub.blue =90 .degree.
In the above two embodiments, the distance between two aligners is 5
microns, and the wavelength of red, green and blue light is 650, 550 and
450 nm, respectively, and .DELTA.n is 0.078. The rotation angle of Liquid
Crystal molecules .theta. can be calculated from red, green and blue light
to be 144, 127 and 90 degrees respectively.
Top aligner made this way has taken into consideration the different
rotation angles for different color pixel and has the function of
retardation film for red, green and blue light.
When combining the bottom aligner shown in FIG. 6(J) and top aligner shown
in FIG. 6(Q), the embodiment 1 is formed, as shown in FIG. 6(U). When
combining the bottom aligner shown in FIG. 6(K) and top aligner shown in
FIG. 6(S), the embodiment 2 is formed, as shown in FIG. 6(V).
By using transparent light-sensitive photoresist as pre-etched aligner, the
process steps (2) and (3) shown in FIG. 6(B) can be combined into a single
step and the extra steps of photoresist coating and non-selective etchback
can be eliminated. In steps shown in FIGS. 6(H), 6(I), 6(P) and 6(R),
direct exposure of the transparent photoresist used as pre-etched aligner
can be used to simplify the process. This simplified process yields the
same structure and property, and is similar to the process described in
FIGS. 6(A) through 6(V) except that the extra steps of photoresist coating
and non-selective etchback have been eliminated.
FIG. 7, shows the top view of the LCD aligner of this invention. The top
aligner for different color pixel whose line has different directions.
This variation of the angle difference of the lines in different
directions is used to achieve retardation for light of different colors.
FIG. 8 shows the cross-sectional representation of the LCD aligner of the
first and second embodiments. Each colored pixel, (R=red, G=green or
B=blue) is divided into two or more domains (domain 4, domain 5, . . . )
and the pretilt angles 12 of the lines in each domain are different.
Comparing with conventional technology, this invention has following
advantages:
(1) To make the LCD aligner by using conventional rubbing method requires
at least three steps to make multi-domain aligner. However only two
lithography steps are needed by the method disclosed of this invention:
one on each electrode. This simplifies the process and reduces the cost.
(2) Conventional technology requires one or two extra retardation film(s).
The aligner of this invention has the special property of retardation
built in and eliminates the step to make the extra retardation film as
well as the requirement of extra material and equipment.
(3) Multi-domain aligner and retardation effect are made in one single step
simultaneously by the lithography method disclosed in this invention.
(4) Using the LCD aligner of this invention can significantly improve the
contrast ratio and reduce the color interference and extend the viewing
angle.
While the invention has been particularly shown an described with reference
to the preferred embodiments thereof, it will be understood by those
skilled in the art that various changes in form and details may be made
without departing from the spirit and scope of the invention.
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